Fibonnaci sequence in PureScript

fib.js

var main = function __do() {
  var _4 = 1;
  var _3 = 1;
  return (function () {
    while (_ps.Prelude.pure(_ps.Prelude.applicativeFromMonad(_ps.Control_Monad_Eff.monadEff({})))(true)()) {
      (function __do() {
        var _2 = _4;
        var _1 = _3;
        _3 = _2 + _1;
        _4 = _1;
        return _ps.Debug_Trace.print(_ps.Prelude.showNumber({}))(_1)();
      })();
    };
    return {};
  })();
};

fib.purs

module Main where

import Control.Monad.Eff
import Control.Monad.ST

main = runST (do
  n1 <- newSTRef 1
  n2 <- newSTRef 1
  whileE (pure true) $ do
    n1' <- readSTRef n1
    n2' <- readSTRef n2
    writeSTRef n2 $ n1' + n2'
    writeSTRef n1 n2'
    Debug.Trace.print n2')